A heat-conductive sheet is disposed between an electronic component or the like serving as a heat generating source and a heat sink such as a radiator plate and a casing, for improving heat dissipation properties of the electronic component.
Such a heat-conductive sheet is required to have adhesiveness and flexibility from the viewpoint of workability when assembling an electronic component and a heat sink using the heat-conductive sheet. Since a material having high flexibility is strong in adhesiveness, an electronic component to which the heat-conductive sheet is adhered likely cannot be separated from a heat dissipating component such as a heat sink.
When adhesiveness is strong, workability when removing an electronic component from a heat dissipating component is poor. Furthermore, even if an electronic component can be removed, it is difficult to determine on which of an electronic component or a heat dissipating component the heat-conductive sheet remains.
On the other hand, from the viewpoint of reworkability when correcting misalignment caused during assembling of an electronic component and a heat dissipating component or when reassembling an electronic component and a heat dissipating component which have been once assembled and thereafter disassembled for any reason, it is said that a heat-conductive sheet preferably has high adhesiveness on one side while having low adhesiveness on the other side.
Under such circumstances, when a heat-conductive sheet is formed from a silicone rubber and a heat-conductive filler, there has been proposed performing a non-adhesion treatment by irradiating the surface of the heat-conductive sheet with ultraviolet rays (Japanese Patent No. 3498823).
Furthermore, there has been proposed, in an adhesive heat-conductive sheet including an acrylic-based polyurethane resin containing a non-functional acrylic polymer and a heat-conductive filler, coating a front surface layer and a back surface layer overlapping each other, each having a different formulating ratio of the acrylic-based polyurethane resin to the non-functional acrylic polymer, thereby the adhesiveness is different between the front and back of the adhesive heat-conductive sheet (JPA 2010-93077).
However, performing ultraviolet irradiation for lowering the adhesiveness of one surface of the heat-conductive sheet as disclosed in Japanese Patent No. 3498823 causes deterioration of the layer responsible for heat-conductivity.
Furthermore, as disclosed in JPA 2010-93077, when coating a front surface layer and a back surface layer overlapping each other, each having a different formulating ratio of the acrylic-based polyurethane resin to the non-functional acrylic polymer, the front surface layer and the back surface layer are likely to be mixed together. Therefore, it is difficult to adjust the adhesivenesses of the front surface layer and the back surface layer as required.
There is a method of providing a non-adhesive tack-free layer on one surface of a heat-conductive layer having adhesiveness thereby to obtain a two-layer structure. An example of such a method includes making a tack-free layer including a large amount of a heat-conductive filler so as to reduce the adhesion of the surface, or disposing a substrate material such as paper or a film on one surface so as to reduce the adhesion of the surface.
However, a heat-conductive sheet obtained in such a method has a problem that the flexibility of the heat-conductive sheet deteriorates, and heat-conductivity decreases, thereby reducing the performance of the heat-conductive sheet.
Furthermore, deformability (stretchability) caused by stress such as bending and stretching applied during a peeling operation is different between the coat of the tack-free layer and the heat-conductive layer which is required to have flexibility. Therefore, when adhesiveness between the tack-free layer and the heat-conductive layer is insufficient, interfacial fracture is easily caused.
In particular, the tack-free layer including a resin having a high glass transition temperature for reducing tack (tackiness) is poor in adhesion with the heat-conductive layer, and thus, there is a problem that the tack-free layer and the heat-conductive layer are peeled from each other.